The present invention relates to synthetic amorphous precipitated silicas, and more particularly, to a novel process for producing synthetic precipitated silicas and silicates having new and improved physical and chemical properties and which have particular use as an abrasive agent in toothpaste compositions.
As known in the art, finely divided amorphous precipitated silicic acid pigments and certain zeolitic type alumino silicates can be prepared by the acidulation of an aqueous silicate solution with an acid or a salt of the acid, such as aluminum sulfate. Such products are commercially available being sold, for example, under the trademarks "Zeo"; "Zeolex" and "Arogen" by the J. M. Huber Corporation. Specific examples of these products as well as methods for their preparation are disclosed in U.S. Pat. Nos. 2,739,073; 2,843,346 and 3,582,379.
In general, the nature or characteristics of the above discussed precipitated silicic acid (sometimes herein referred to as silicas) and silicate pigments depend, in part, on the chemistry of the silicate solution (specifically the SiO.sub.2 /Na.sub.2 O ratio of the silicate) as well as the reaction conditions employed (precipitating pH, etc.). However such known pigments are characterized by, and have, the following properties: High structure, high wet cake moisture content, high oil absorption, low valley abrasion, high surface area and low pack density. In this regard, and due to the properties such as high oil absorption, high surface area, etc., the pigments have been widely and successfully used as reinforcing pigments in rubber, in paints, in the manufacture of paper, as moisture conditioners and the like.
However, the high wet cake moisture content is disadvantageous in that the drying and filtration rates are increased thus increasing the overall cost of the final product. For example, in the conventional production of silicic acid pigments as defined above, the wet cake moisture content of the product (following filtration of the precipitated reaction mass) is approximately 82%. This means that there can be recovered only 18 parts of dry pigment from 100 parts of wet cake.
Further, and very importantly, the low abrasiveness of known silica and silicate pigments renders then unsuitable for many uses. For example, it is well known that conventional synthetic precipitated silicates are unsuitable as polishing and abrasive agents in toothpaste compositions. See German Pat. No. 974,958; French Pat. No. 1,130,627; British Pat. No. 995,351; Swiss Pat. No. 280,671 and U.S. Pat. No. 3,250,680. In this regard, it is disclosed in U.S. Pat. No. 3,538,230 that known amorphous silicas such as precipitated silicas, pyrogenic silicas and aerogels are unsuitable for dentifrice use because of their initial small particle size and because of the ease in which they break down into small particle sizes which result in poor cleaning ability.
Further, conventional silicas and amorphous precipitated alumino silicates, such as "Zeolex" and "Arogen", cannot be used for a clear gel toothpaste because of their high refractive index (1.55) and because they lack the needed abrasive and polishing characteristics when added to the toothpaste base composition. Clear gel toothpaste contains a high percentage of abrasive and polishing agent in the toothpaste formula. The major function of the abrasive and polishing agent is to remove stains, food debris and bacterial plaque from the human tooth surface. Ideally the polishing agent should provide a maximum cleaning action at acceptable abrasion levels and must be compatible at high loadings of 15% up to 50% with other toothpaste formula ingredients. Thus the above noted silicas and alumino silicates are unsuitable for clear gel toothpastes, (such as the product sold under the trademark "Close-Up" by Lever Brothers), because they cannot be added at high loadings of 15% and above in a typical toothpaste composition. Also, because of their high oil absorption and high refractive index (1.55) known precipitated pigments thicken up the dentifrice composition and impart undesirable opacity to the base paste resulting in an unacceptable product.
More recently, however, and before turning to the details of the present invention, there has been developed, as disclosed in detail in U.S. Pat. No. 3,893,840, a process for producing siliceous pigments having improved physical and chemical properties including low structure, a low wet cake moisture content, high abrasion, low oil absorption and high pack densities. In accordance with the teaching of this application, the pigments are prepared by acidulating a solution of an alkali metal silicate with an acid until precipitation just begins. At this point the reaction mass is aged for a period of time and thereafter the acid addition is continued until the precipitated product is obtained. In accordance with one method embodiment disclosed by this application an adduct material, such as aluminum, is pre-mixed with the acid to control the refractive index of the pigment so that same may be employed in clear toothpaste composition.
The present invention is directed to a further method for producing silicic acid and silicate pigments having the aforementioned improved properties.